1. Field of the Invention
The present invention relates to a press working control method and a control system for controlling the die-forming operation of a hydraulic press in die-forming a work piece such as a resin plate and, more specifically, to a press working control method and a control system for controlling a die-forming process so that the movable die is maintained in a level position and pressure is applied uniformly to the workpiece during the die-forming process.
2. Description of the Prior Art
In die-forming a workpiece such as a resin plate on a hydraulic press, it is important, for securing satisfactory reproducibility, to maintain the movable die at a high parallel accuracy and to apply pressure uniformly to the workpiece during a period from the moment of contact of the movable die with the workpiece to the completion of the die-forming process. However, the conventional press which depends on mechanical rigidity of the construction for accurate press working is unable to maintain the die at a high parallel accuracy of, for example, 0.05 mm and to maintain the working pressure acting on the workpiece at a fixed level.
An improved press working control method eliminating those drawbacks is disclosed in Japanese Patent Provisional Publication (Kokai) No. 60-30323. According to this improved press working control method, a press slide carrying a movable die is balanced accurately by balancing mechanisms provided on the bed of the press, the supporting force of each balancing mechanism, namely, load on each balancing mechanism, is detected, the detected supporting force is compared with a predetermined reference force, the lowering speed of the press slide is reduced when the detected supporting force is greater than the reference force or is increased when the supporting force is smaller than the reference force, in order to make the balancing mechanisms always exert an effective counter force on the press slide so that the press slide, hence the movable die, is supported at a high parallel accuracy and a fixed pressure is applied to the workpiece.
However, whereas the parallel accuracy is quantified by height, the dynamic parallel accuracy of the press slide is controlled on the basis of the difference between the respective speeds of the balancing mechanisms. Accordingly, the parallel accuracy of the press slide is deteriorated due to the inherent delay of the control action. Furthermore, since the respective speeds of the balancing mechanisms are reduced synchronously, the dynamic parallel accuracy of the press slide is deteriorated when the workpiece is dislocated from a correct position as shown in FIG. 11(a) or when the viscosity of the workpiece varies by position in the workpiece as shown in FIG. 11(b).
On the other hand, the ram speed is regulated to apply a fixed pressure to the workpiece. Therefore, two control loops, namely, a control loop for controlling the speed of the balancing mechanisms and a control loop for controlling the ram speed, need to be executed by a single operating means. Consequently, both the objects of the press working control method, namely, maintaining the press slide at a high parallel accuracy and applying a fixed pressure to the workpiece, interfere with each other due to the difference between the two control loops in timing.